#162 Add collapsible docs nav sections

Summary

• add a small custom nav script that turns top-level Mintlify sidebar headers into collapsible controls
• keep the current page section expanded while collapsing inactive sections by default
• add chevron/focus styling and regression coverage for click and keyboard toggles

Tests

• node --test scripts/*.test.mjs
• node scripts/sweep-artifacts.mjs --check

Notes

• Verified locally in Mintlify preview: Concepts page keeps Concepts open while AI in Runway/Data in collapse; AI pages keep AI in Runway open.

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Summary by cubic

Make top-level docs sidebar sections collapsible to reduce noise and improve navigation. The current section stays open, and user choices are remembered.

• New Features
  • Added nav-collapse.js to make Mintlify sidebar headers toggleable via click and keyboard, with ARIA attributes, active section kept open, and state persisted in localStorage.
  • Chevron indicator and focus styling added in styles/nav.css.
  • Unit tests added in scripts/nav-collapse.test.mjs for collapse behavior and keyboard/click toggles.
  • Updated scripts/sweep-artifacts.mjs to include nav-collapse.js as a reference file.